In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens having a numerical aperture (NA) of 1.0 or greater. Theoretically, it is possible to increase the NA to nearly 1.44. It was initially recognized that the resolution could be degraded and the focus be shifted by a variation of water's refractive index with a temperature change. The problem of refractive index variation could be solved by controlling the water temperature within a tolerance of 1/100° C. while it was recognized that the impact of heat from the resist film upon light exposure drew little concern. There was a likelihood that micro-bubbles in water could be transferred to the pattern. The risk of bubble generation could be obviated by thorough deaeration of water, and the risk of bubble generation from the resist film upon light exposure is substantially nil. At the initial phase in 1980's of the immersion lithography, a method of immersing an overall stage in water was proposed. Later proposed was a partial-fill method of using a water feed/drain nozzle for introducing water only between the projection lens and the wafer so as to comply with the operation of a high-speed scanner. In principle, the immersion technique using water enabled lens design to a NA of 1 or greater. In optical systems based on traditional refractive index materials, this leads to giant lenses, which would deform by their own weight. For the design of more compact lenses, a catadioptric system was proposed, accelerating the lens design to a NA of 1.0 or greater. A combination of a lens having NA of 1.35 with strong resolution enhancement technology enables the mass-scale production of 45-nm node devices.
One recent approach for the manufacture of CMOS devices is to effect ion implantation on a substrate through a KrF resist pattern as a mask in order to form p- and n-wells in the substrate. As the resist pattern is reduced in size, a replacement by an ArF resist pattern is in progress. In order to carry out ion implantation, the substrate surface must be exposed through space areas in the resist film pattern. If a bottom antireflective coating (BARC) layer is present beneath the resist film, ions can be trapped by the BARC layer. However, if the photoresist film is patterned in the absence of the BARC layer, then standing waves are generated by substrate reflection, whereby the resist pattern after development has substantially ridged sidewalls. For the purpose of smoothening such ridges due to standing waves, it is believed effective to enhance acid diffusion by using a photoacid generator (PAG) capable of generating a low molecular weight acid which is prone to diffuse and performing PEB at higher temperature. As long as the size at which the resist film subject to ion implantation is resolved by the KrF lithography is in the range of 200 to 300 nm, it is not recognized that resolution is degraded by enhancement of acid diffusion. However, when the size at which the resist film subject to ion implantation is resolved by the ArF lithography is reduced below 200 nm, undesirably the enhancement of acid diffusion can cause degradation of resolution and increase the proximity bias.
The most traditional means for preventing generation of standing waves is a dyed resist material for forming a photoresist film which is absorptive in itself. The study on that means started from the novolak resist materials for i and g-line exposure. As the absorptive component which can be used in the ArF lithography, a study was made on the introduction of benzene ring into a base polymer and the use of an additive having benzene ring. However, it is impossible for the absorptive component to completely prevent standing waves. If the component is made more absorptive, standing waves are reduced, but the cross-sectional profile of a resist pattern can be tapered into a trapezoidal shape.
Since naphthalene ring has higher etch resistance than benzene ring, it was attempted to apply naphthalene ring to resist polymers (see Patent Documents 1 and 2). In particular, naphthalene ring and acenaphthene having a hydroxyl group have the advantage of improved adhesion to substrates over the use of only lactone ring as the adhesive group.
Patent Document 3 discloses an ion implantation-amenable resist composition comprising a methacrylate polymer having acid labile groups in blend with a cresol novolak resin, from which a pattern is formed by the KrF lithography. Despite the economic merit of using an inexpensive cresol novolak resin, this resist composition cannot be applied to the ArF lithography because of the strong absorption of the cresol novolak resin.
Patent Document 4 discloses an ion implantation-amenable resist composition comprising hydroxynaphthalene or dihydroxynaphthalene optionally substituted with an acid labile group. The addition of the monomeric component improves coverage on stepped substrates. However, since naphthalene is sublimate, a problem can arise that naphthalene component evaporates during bake and deposits on top of the hot plate.
Patent Document 5 discloses an ion implantation-amenable resist composition comprising naphthalene ring or acenaphthene. A copolymer of a naphthalene or acenaphthylene monomer, an acid labile group-containing monomer, and a monomer having a lactone adhesive group is used as the base resin. When naphthalene has a hydroxyl group, substrate adhesion is improved, but not step coverage.
Patent Document 6 discloses a thick-film resist composition comprising a blend of a polymer of t-butyl (meth)acrylate and a novolak resin. A novolak resin using dihydroxynaphthalene is exemplary. Since the acid labile group is t-butyl, the lithography performance is inferior due to a shortage of dissolution inhibition. In the ion implantation application, the resist film may have poor mask function against ion implantation.